1. Field of the Invention
The present invention relates to a high-strength polyolefin fiber having an improved initial elongation and a process for the preparation thereof.
2. Description of the Related Art
It is known that a molecularly oriented shaped body having high elastic modulus and high tensile strength is obtained by shaping ultra-high-molecular-weight polyethylene into a fiber, a tape or the like and drawing the shaped body. For example, Japanese Patent Application Laid-Open Specification No. 15408/81 discloses a process in which a dilute solution of ultra-high-molecular-weight polyethylene is spun and the obtained filament is drawn. Futhermore, Japanese Patent Application Laid-Open Specification No. 130313/84 discloses a process in which ultra-high-molecular-weight polyethylene is melt-kneaded with a wax and the kneaded mixture is extruded, cooled, solidified and then drawn. Moreover, Japanese Patent Application Laid-Open Specification No. 187614/84 discloses a process in which a melt-kneaded mixture as mentioned above is extruded, drafted, cooled, solidified and then drawn.
If ultra-high-molecular-weight polyethylene is shaped into a fiber and the fiber is strongly drawn, the elastic modulus and tensile strength are increased with increase of the draw ratio. This drawn fiber has good mechanical properties such as high elastic modulus and high tensile strength and is excellent in light weight characteristic, water resistance and weatherabililty. However, this drawn fiber is still insufficient and defective in that the initial elongation is large and the creep resistance is poor.
The initial elongation is a phenomenon which is peculiarly and commonly observed in organic fibers, and this phenomenon is observed even in rigid high polymers such as a Kevlar fiber (wholly aromatic polyamide fiber). Especially in the above-mentioned polyethylene fiber having high elastic modulus and high strength, the initial elongation is so large as about 1% at normal temperature, and a high elastic modulus cannot be sufficiently utilized in the field of, for example, a composite material or the like. More specifically, influences by this large initial elongation are serious in fiber-reinforced resin composite materials, tension members (optical fiber cords) and the like.